This invention relates to improvements in optical systems of the type conventionally employed, for example, in intruder detection systems of the passive infrared variety.
Conventional passive infrared intrusion detection systems typically comprise a multiple field-of-view optical system for directing infrared radiation (IR) emanating from any one of a plurality of discrete fields of view onto a single pyroelectric detector, or a closely spaced pair of such detectors. See, for example, the optical systems disclosed in U.S. Pat. No. 3,703,718 issued in the name of H. L. Berman. The optical systems disclosed in the Berman patent comprise, in general, a plurality of discrete, spherical mirror segments having a common focal point. Each mirror segment is inclined with respect to the other segments to provide an IR detector located at the common focal point with a plurality of discrete, sector-shaped fields of view. As an IR source (e.g. a human being) moves into and out of these fields of view, a sudden change in the level of IR radiation is sensed by the detector and an alarm is sounded.
Aside from being relatively costly to manufacture and difficult to optically align and maintain in focus, multi-field-of-view optical systems of the type disclosed in the Berman patent have other drawbacks when used in passive IR detection systems. For example, in installing such systems, it is often desirable to selectively mask one or more of the reflective segments to prevent a false alarm-producing source (e.g. a heating duct or light bulb) from being within one or more of the multiple fields of view. This problem could be alleviated by simply applying a masking material to the segment(s) which would otherwise focus the false alarm-providing source on the IR detector. But, owing to their non-transparent and reflective nature, these mirror segments must be positioned behind the sensor element; hence, they are not readily accessible for the purpose of applying such masking material.
Another undesirable characteristic of such multifaceted reflective optical systems is that they are typically of relatively short focal length, a property which allows the overall dimensions of the detector housing to be minimized. Unfortunately, as the focal length diminishes, the field of view of each reflector increases, which, in turn, reduces the sensitivity of the system. While it is known to optically fold reflective optical systems by the use of or additional mirrors, such additional elements are costly; moreover, they add substantial optical losses to the system.
A possible solution to the aforementioned problems with multifaceted reflective optical systems is disclosed in U.S. Pat. No. 4,275,303, issued to P. H. Mudge. Such an optical system substitutes an array of Fresnel lenses for the multiple mirror segments, each Fresnel lens being tilted with respect to the others so as to have its own discrete field of view. A refractive system such as this allows the focusing elements to be positioned in front of the detector, and thereby facilitates the task of selective masking. Moreover, such an "up front" optical system can be optically folded without incurring substantial optical loss, and allows easy substitution of one Fresnel lens array for another to achieve variations in the pattern of protection. While the Fresnel lens approach overcomes many of the disadvantages associated with the above-mentioned reflective-type optical systems, it has certain disadvantages of its own. For example, assuming the desirability of (a) being able to adjust the position of the Fresnel lens relative to the detector housing so as to alter the directions in which the several fields-of-view are aiming, and (b) having a fixed IR-transmitting window on the detector housing to prevent dust, wind currents, etc., from causing false alarms, it is necessary to use two separate IR-transmitting elements in such a system; i.e., a movable Fresnel lens and a fixed exterior window. This requirement, of course, adds to the system cost and introduces optical losses which adversely affect sensitivity. Still nother drawback of such Fresnel systems is that each lens element must be precisely positioned and angularly disposed with respect to the other lens elements so as to share a common focal point. In this regard, they are no easier to align and maintain in focus than the aforementioned reflective optical systems. Moreover, should it be desirable to substitute one lens array for another (e.g. to eliminate a damaged lens or to alter the pattern of the fields of view), it is necessary to realign and refocus the entire optical system.